README: Update ver env var description

Commit 155cb010 replaced the read-only property of the ver env var
with an auto-restoring behavior. Update the README file accordingly.
Signed-off-by: NBenoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Wolfgang Denk <wd@denx.de>

README

@@ -907,7 +907,8 @@ The following options need to be configured:
 		If this variable is defined, an environment variable
 		named "ver" is created by U-Boot showing the U-Boot
 		version as printed by the "version" command.
-		This variable is readonly.
+		Any change to this variable will be reverted at the
+		next reset.
 
 - Real-Time Clock: